Robust antiferromagnetic spin waves across the metal-insulator transition in hole-doped BaMn2As2

Abstract

BaMn2As2 is an antiferromagnetic insulator where a metal-insulator transition occurs with hole doping via the substitution of Ba with K. The metal-insulator transition causes only a small suppression of the Néel temperature (TN) and the ordered moment, suggesting that doped holes interact weakly with the Mn spin system. Powder inelastic neutron scattering measurements were performed on three different powder samples of Ba1−xKxMn2As2 with x =0, 0.125 and 0.25 to study the effect of hole doping and metallization on the spin dynamics of these compounds. We compare the neutron intensities to a linear spin wave theory approximation to the J1 − J2 − Jc Heisenberg model. Hole doping is found to introduce only minor modifications to the exchange energies and spin gap. The changes observed in the exchange constants are consistent with the small drop of TN with doping. INTRODUCTIONThe parent compounds of unconventional superconductors are typically antiferromagnetic (AFM), although they may be initially metals (as in the iron pnictides [1]) or Mott insulators (as in the copper oxide superconductors [2]). In either case, chemical substitution is often employed to destabilize the AFM ordered state and give rise to a superconducting ground state over some composition range. For the iron arsenides, they are already metallic and adding charge carriers serves to modify the Fermi surface and destabilize nesting-driven spin-density wave AFM order, resulting in superconductivity. In BaFe 2 As 2 , electrons or holes can be added via chemical substitutions such as Ba(Fe 1−x M x ) 2 As 2 with M = Co or Ni [3][4][5][6][7][8][9] and Ba 1−x K x Fe 2 As 2 [10], respectively.On the other hand, the effect of chemical substitutions in the copper oxides, such as La 2 CuO 4 , are twofold. Since they are insulators, chemical substitutions (such as La 2−x Sr x CuO 4 ) must both metallize the system and disrupt long-range AFM order in order to make the conditions favorable for superconductivity. In other words, both a metal-insulator transition and suppression of AFM order are necessary for superconductivity to appear in the cuprates.In attempting to find some commonality between the arsenides and cuprates, we search for other compounds that bridge these two systems. One possible system is BaMn 2 As 2 which shares the same crystal structure as BaFe 2 As 2 . Similar to cuprates, BaMn 2 As 2 is a quasitwo-dimensional AFM insulator possessing a square lattice of magnetic moments that order into a G-type AFM (checkerboard) pattern [11]. Also similar to the cuprates, a metal-insulator transition can be induced in BaMn 2 As 2 by replacing small amounts (a few percent) of Ba with K which effectively adds hole carriers [12,13]. Unlike the cuprates, the moments on Mn are large (S ≈ 2 to 5/2). In conjunction with large magnetic interactions (with a Néel transition temperature of T N = 625 K) [11,14,15], both neutron diffraction [16] and 75 As nuclear magnetic resonance [17] find that long-range AFM ordering is robust with hole doping, even at K concentra...

Studies of spin dynamics in 122 transition metal arsenides using inelastic neutron scattering technique

“…2.9. This indicates that the G-type order of BaMn 2 As 2 is robust against the electron-doping or metallicity and is inconsistent with the expectations based upon the significant hybridization between Mn and As orbitals in BaMn 2 As 2 [51].…”

“…Again, defining anisotropy parameter η = J 1 /2J 2 , which is also the definition of the frustration parameter, and an average correlation length ξ 2 /a 2 = J 2 χ o to obtain [51].…”

“…The condition for three different magnetic ground states are given in the Table 2.1. [51] based upon the ratio J 2 J 1 obtained from the fits to the inelastic neutron scattering data. The figure is created using the figure in the Ref.…”

Spin dynamics in antiferromagnetic oxypnictides and fluoropnictides: LaMnAsO, LaMnSbO, and BaMnAsF

“…This is similar to a Warren lineshape which corresponds to the powder averaging of rod of scattering. This behavior can be contrasted with similar cuts in the INS of the more 3D-like BaMn 2 As 2 for which the scattering is modulated with peaks that are near (H0L) reflections [15].…”

“…For LaMnAsO we identify significant magnetic INS contribution from MnO that is present as an impurity phase (for details on the magnetic INS contribution of MnO polycrystalline see Ref. [15]).…”